The present invention relates to a sheet finishing apparatus that creates booklets of sheets formed with images by an image forming apparatus, such as a printer, and then discharges the booklets therefrom, or that folds the sheets twice or three times according to a desired method of filing. The invention also relates to an image forming apparatus equipped with the sheet finishing apparatus.
A sheet processing apparatus, such as the type described above, is generally used as an apparatus for folding sheets discharged from a printing machine or printer, or for conveying the folded sheets to a finishing apparatus for binding. Many of such apparatuses are connected to an image forming apparatus, such as a printer. The apparatuses are widely used as a printing system that can perform continuous operations of processes from printing to booklet-making by discharging sheets after a folding process, or feeding sheets to a finishing apparatus for a binding procedure, such as stapling.
Conventionally, the type of sheet finishing apparatuses described above have a pair of folding rollers in mutual contact in a path for conveying a sheet. The sheet is folded into two by feeding a folding position of the sheet conveyed in the conveyance path nipped between these folding rollers. Still further, the sheet can be folded into three or four by feeding rollers established at a downstream side. When folding a sheet using a pair of folding rollers, it is necessary to accurately calculate the folding position of the sheet. If the sheet folding position is incorrect or skewed, accurate folding is not possible, and the sheet can become wrinkled.
In the prior art, Japanese Patent Publication (JP) 2004-99199 discloses a method of inserting a folding blade at a folding position, while the leading edge of a sheet fed at a conveyance path engages a stopper and is stationary, so as to press the folding position of a sheet into folding rollers. In this method, the sheet is temporarily stopped in the conveyance path, at which point the knife-edge-shaped folding blade presses at a predetermined folding position of the sheet positioned at a stopper. This method for positioning the folding position of a sheet at the folding rollers is comparatively accurate. But, this type of folding process requires additional time because the sheet is temporarily stopped in the conveyance path, and the folding blade must move into a contact position with the folding rollers from a position retracted from the conveyance path.
Also disclosed in JP 2004-99199 is a structure for guiding a sheet with one end folded from the folding rollers into a conveyance path, and for folding the other end of the sheet to form a so-called letter fold at the conveyance path. This structure employs a stopper for engaging an end (i.e., a leading edge) of the sheet in the conveyance path, and two folding rollers for folding the other end of the sheet. With this structure, the leading edge of the sheet folded by a first pair of folding rollers engages the stopper. Then, the trailing edge of the sheet is fed from the first pair of folding rollers into a second pair of folding rollers. That is, the system calculates the folding position by pressing a stationary sheet positioned at a stopper into the first pair of folding rollers using a folding blade, and calculates the folding position by feeding the trailing edge of the sheet to the second pair of folding rollers, while the leading edge is engaged and pressing through a pair of folding rollers.
But, a method that calculates a folding position by engaging a leading edge of a sheet at a stopper requires that the stopper position be moved to adjust both for the sheet length, and for the method of folding the sheet, such as two or three folds.
Japanese Patent Publication (JP) 2003-118932 arranges conveyance rollers separated a distance on a conveyance path, and disposes folding rollers between these rollers in front and behind. Then, while one roller nips and holds the sheet stationary, another roller feeds the sheet into the folding rollers to perform the folding process. To apply a second fold to the sheet, second folding rollers are arranged at a discharge side, and conveyance rollers are arranged at a downstream side of these folding rollers. These conveyance rollers nip and stop a sheet folded by the first folding rollers, and bend to feed the sheet in this state into the second folding rollers. In other words, the first and the second folding rollers are arranged sequentially in the conveyance path. The sheet is fed into the folding rollers while a predetermined position of a leading edge side in a direction of sheet conveyance is nipped and stopped by conveyance rollers, and the sheet is bent. After the folding rollers feed in the sheet, the folding position is formed by reversing the conveyance rollers positioned at a downstream side.
To apply two or three folds to a sheet discharged from an image forming apparatus, such as the apparatus described in JP 2004-99199, a conveyance path normally includes first and second folding rollers. Either a leading edge of the sheet is engaged by a stopper to calculate the folding position, or, as described in JP 2003-118932, forward and reverse driving rollers for nipping a sheet are disposed at a downstream side of first and second folding rollers. The folding position is determined by stopping the rollers.
However, to apply a second or more folds to a sheet, the methods for calculating the folding positions are different, as described below, for the first and the second folding rollers. A method of engaging a leading edge of a sheet at a stopper is used to calculate a folding position for the first folding rollers. In that method, the stopper moves to a prescribed position in the path according to the sheet length. Then, the folding proceeds, such as, for example, applying two or three folds. Compared to the stopper for calculating the folding position of the second folding rollers where one end is already folded, the stopper for calculating the folding position of the first folding rollers must be movable for a longer distance, making the device more complex. At the same time, the distance between the first folding rollers and stopper change according to the sheet size. When a long sheet is supportingly guided by a conveyance guide for a long distance, it becomes unstable, which invites skewing problems in the folding position (i.e., the position fed into the first folding rollers) or wrinkle formation in the sheet.
Conversely, by using conveyance rollers to position a folding position at the first folding rollers, a comparatively stable folding position can be attained regardless of the length of a sheet. However, the following problems occur when positioning a folding position using conveyance rollers and employing the second folding rollers.
In other words, because one end of a sheet guided to the second folding rollers is already folded, the folded sheet is fed in one direction while conveyance rollers nip the sheet. At a predetermined position these rollers stop, and feed the sheet by reversing when the trailing edge of the sheet is being fed into the second folding rollers. For that reason, the pair of rollers drives in a forward and a reverse direction while nipping the folded sheet. When the folding position of the once folded sheet is nipped by the rollers, a double fold occurs, causing wrinkles to be formed in the sheet fold.
Thus, in the prior art, either a stopper is engaged for the folding position of the first or the second folding rollers, or a sheet is controlled by conveyance rollers. Use of these prior art methods can, however, result in the above-described problems.
In view of the aforementioned problems associated with the prior art, a first object of the present invention is to provide a sheet finishing apparatus having both a compact and simple structure, and having a mechanism for accurately positioning a sheet at the first folding rollers when applying two or three folds to a sheet. A related object of the invention is to provide a sheet finishing apparatus that can apply comparatively accurate folds to the sheet with a mechanism for positioning a folded sheet at the second folding rollers, yet without destroying a previous folded position or causing wrinkles in the fold.
A second object of the present invention is to provide a sheet finishing apparatus that can easily calculate a folding position according to folding conditions, such as, for example, sheet folding procedures or sheet length. A related object of the invention is to provide a sheet finishing apparatus in which discrepancies in sheet or conveyance do not affect the folding position.
A third object of the present invention is to provide an image forming apparatus equipped with a sheet finishing apparatus that accurately folds sheets.
Further objects and advantages of the invention will be apparent from the following description of the invention and the associated drawings.